Reference sections for some of the new things in Dotty

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -431,11 +431,18 @@ object desugar {
     // For all other classes, the parent is AnyRef.
     val companions =
       if (isCaseClass) {
+        def extractType(t: Tree): Tree = t match {
+          case Apply(t1, _) => extractType(t1)
+          case TypeApply(t1, ts) => AppliedTypeTree(extractType(t1), ts)
+          case Select(t1, nme.CONSTRUCTOR) => extractType(t1)
+          case New(t1) => t1
+          case t1 => t1
+        }
         // The return type of the `apply` method
         val applyResultTpt =
           if (isEnumCase)
             if (parents.isEmpty) enumClassTypeRef
-            else parents.reduceLeft(AndTypeTree)
+            else parents.map(extractType).reduceLeft(AndTypeTree)
           else TypeTree()
 
         val parent =

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -443,13 +443,19 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
     tree.tpt match {
       case templ: untpd.Template =>
         import untpd._
-        templ.parents foreach {
+        var templ1 = templ
+        def isEligible(tp: Type) = tp.exists && !tp.typeSymbol.is(Final)
+        if (templ1.parents.isEmpty &&
+            isFullyDefined(pt, ForceDegree.noBottom) &&
+            isEligible(pt.underlyingClassRef(refinementOK = false)))
+          templ1 = cpy.Template(templ)(parents = untpd.TypeTree(pt) :: Nil)
+        templ1.parents foreach {
           case parent: RefTree =>
             typedAheadImpl(parent, tree => inferTypeParams(typedType(tree), pt))
           case _ =>
         }
         val x = tpnme.ANON_CLASS
-        val clsDef = TypeDef(x, templ).withFlags(Final)
+        val clsDef = TypeDef(x, templ1).withFlags(Final)
         typed(cpy.Block(tree)(clsDef :: Nil, New(Ident(x), Nil)), pt)
       case _ =>
         var tpt1 = typedType(tree.tpt)

docs/docs/index.md

@@ -14,6 +14,14 @@ the code, setup Dotty with your favorite IDE and more!
 Contents
 -------
 
+* Dotty Language Reference
+    - [Intersection Types](reference/intersection-types.md)
+    - [Union Types](reference/union-types.md)
+    - [Trait Parameters](reference/trait-parameters.md)
+    - [Enumerations](reference/enums.md)
+    - [Algebraic Data Types](reference/adts.md)
+    - [Enum Translation](reference/desugarEnums.md)
+    - [By-Name Implicits](reference/implicit-by-name-parameters.md)
 * Usage
     - [Migrating from Scala 2](usage/migrating.md): migration information
     - [Dotty projects with cbt](usage/cbt-projects.md): using cbt

docs/docs/reference/adts.md

@@ -0,0 +1,122 @@
+---
+layout: doc-page
+title: "Algebraic Data Types"
+---
+
+
+The `enum` concept is general enough to also support algebraic data
+types (ADTs) and their generalized version (GADTs). Here's an example
+how an `Option` type can be represented as an ADT:
+
+```scala
+enum Option[+T] {
+  case Some[+T](x: T)
+  case None
+}
+```
+
+This example introduces `Option` enum class with a covariant type
+parameter `T`, together with two cases, `Some` and `None`. `Some` is
+parameterized with a type parameter `T` and a value parameter `x`. It
+is a shorthand for writing a case class that extends `Option`. Since
+`None` is not parameterized it is treated as a normal enum value.
+
+The `extends` clauses that were omitted in the example above can also
+be given explicitly:
+
+```scala
+enum Option[+T] {
+  case Some[+T](x: T) extends Option[T]
+  case None           extends Option[Nothing]
+}
+```
+
+Note that the parent type of `None` is inferred as
+`Option[Nothing]`. Generally, all covariant type parameters of the enum
+class are minimized in a compiler-generated extends clause whereas all
+contravariant type parameters are maximized. If `Option` was non-variant,
+you'd need to give the extends clause of `None` explicitly.
+
+As for normal enum values, the cases of an `enum` are all defined in
+the `enum`s companion object. So it's `Option.Some` and `Option.None`
+unless the definitions are "pulled out" with an import:
+
+```scala
+scala> Option.Some("hello")
+val res1: t2.Option[String] = Some(hello)
+scala> Option.None
+val res2: t2.Option[Nothing] = None
+```
+
+Note that the type of the expressions above is always `Option`. That
+is, the implementation case classes are not visible in the result
+types of their `apply` methods. This is a subtle difference with
+respect to normal case classes. The classes making up the cases do
+exist, and can be unvealed by constructing them directly with a `new`.
+
+```scala
+val res3: t2.Option.Some[Int] = Some(2)
+scala> scala> new Option.Some(2)
+```
+
+As all other enums, ADTs can have methods on both the enum class and
+its companion object. For instance, here is `Option` again, with an
+`isDefined` method and an `Option(...)` constructor.
+
+```scala
+enum class Option[+T] {
+   def isDefined: Boolean
+}
+object Option {
+  def apply[T >: Null](x: T): Option[T] =
+    if (x == null) None else Some(x)
+
+  case Some[+T](x: T) {
+     def isDefined = true
+  }
+  case None {
+     def isDefined = false
+  }
+}
+```
+
+Enumerations and ADTs have been presented as two different
+concepts. But since they share the same syntactic construct, they can
+be seen simply as two ends of a spectrum and it is perfectly possible
+to conctruct hybrids. For instance, the code below gives an
+implementation of `Color` either with three enum values or with a
+parameterized case that takes an RGB value.
+
+```scala
+enum Color(val rgb: Int) {
+  case Red   extends Color(0xFF0000)
+  case Green extends Color(0x00FF00)
+  case Blue  extends Color(0x0000FF)
+  case Mix(mix: Int) extends Color(mix)
+}
+```
+
+## Syntax of Enums
+
+Changes to the syntax fall in two categories: enum classes and cases inside enums.
+The changes are specified below as deltas with respect to the Scala syntax given [here](https://github.com/lampepfl/dotty/blob/master/docs/docs/internals/syntax.md)
+
+ 1. Enum definitions and enum classes are defined as follows:
+
+        TmplDef ::=  `enum' `class’ ClassDef
+                 |   `enum' EnumDef
+        EnumDef ::=  id ClassConstr [`extends' [ConstrApps]]
+                     [nl] `{’ EnumCaseStat {semi EnumCaseStat} `}’
+
+2. Cases of enums are defined as follows:
+
+        EnumCaseStat  ::=  {Annotation [nl]} {Modifier} EnumCase
+        EnumCase      ::=  `case' (EnumClassDef | ObjectDef | ids)
+        EnumClassDef  ::=  id [ClsTpeParamClause | ClsParamClause]
+                           ClsParamClauses TemplateOpt
+        TemplateStat  ::=  ... | EnumCaseStat
+
+
+
+
+

docs/docs/reference/desugarEnums.md

@@ -0,0 +1,165 @@
+---
+layout: doc-page
+title: "Translation of Enums and ADTs"
+---
+
+The compiler expands enum classes and cases to code that only uses
+Scala's other language features. As such, enums in Scala are
+convenient _syntactic sugar_, but they are not essential to understand
+Scala's core.
+
+We now explain the expansion of enums is explained in detail. First,
+some terminology and notational conventions:
+
+ - We use `E` as a name of an enum class, and `C` as a name of an enum case that appears in the companion object of `E`.
+ - We use `<...>` for syntactic constructs that in some circumstances might be empty. For instance `<body>` represents either the body of a case between `{...}` or nothing at all.
+
+ - Enum cases fall into three categories:
+
+   - _Class cases_ are those cases that are parameterized, either with a type parameter section `[...]` or with one or more (possibly empty) parameter sections `(...)`.
+   - _Simple cases_ are cases of a non-generic enum class that have neither parameters nor an extends clause or body. That is, they consist of a name only.
+   - _Value cases_ are all cases that do not have a parameter section but that do have a (possibly generated) extends clause and/or a body.
+
+  Simple cases and value cases are called collectively _singleton cases_.
+
+The desugaring rules imply that class cases are mapped to case classes, and singleton cases are mapped to `val` definitions.
+
+There are eight desugaring rules. Rules (1) and (2) desugar enums and
+enum classes. Rules (3) and (4) define extends clauses for cases that
+are missing them. Rules (5 - 7) define how such expanded cases map
+into case classes, case objects or vals. Finally, rule (8) expands
+comma separated simple cases into a sequence of cases.
+
+1.  An `enum` definition
+
+         enum E ... { <cases> }
+
+    expands to an enum class and a companion object
+
+        enum class E ...
+        object E { <cases> }
+
+2. An enum class definition
+
+       enum class E ... extends <parents> ...
+
+    expands to a `sealed` `abstract` class that extends the `scala.Enum` trait:
+
+       sealed abstract class E ... extends <parents> with scala.Enum ...
+
+3. If `E` is an enum class without type parameters, then a case in its companion object without an extends clause
+
+       case C <params> <body>
+
+    expands to
+
+       case C <params> <body> extends E
+
+4. If `E` is an enum class with type parameters `Ts`, then a case in its
+   companion object without an extends clause
+
+       case C <params> <body>
+
+   expands according to two alternatives, depending whether `C` has type
+   parameters or not. If `C` has type parameters, they must have the same
+   names and appear in the same order as the enum type parameters `Ts`
+   (variances may be different, however). In this case
+
+       case C [Ts] <params> <body>
+
+   expands to
+
+       case C[Ts] <params> extends E[Ts] <body>
+
+   For the case where `C` does not have type parameters, assume `E`'s type
+   parameters are
+
+       V1 T1 > L1 <: U1 ,   ... ,    Vn Tn >: Ln <: Un      (n > 0)
+
+   where each of the variances `Vi` is either `'+'` or `'-'`. Then the case
+   expands to
+
+       case C <params> extends E[B1, ..., Bn] <body>
+
+   where `Bi` is `Li` if `Vi = '+'` and `Ui` if `Vi = '-'`. It is an error if
+   `Bi` refers to some other type   parameter `Tj (j = 0,..,n-1)`. It is also
+   an error if `E` has type parameters that are non-variant.
+
+5. A class case
+
+       case C <params> ...
+
+   expands analogous to a case class:
+
+       final case class C <params> ...
+
+   However, unlike for a regular case class, the return type of the associated
+   `apply` method is a fully parameterized type instance of the enum class `E`
+   itself instead of `C`.  Also the enum case defines an `enumTag` method of
+   the form
+
+       def enumTag = n
+
+   where `n` is the ordinal number of the case in the companion object,
+   starting from 0.
+
+6. A value case
+
+       case C extends <parents> <body>
+
+   expands to a value definition
+
+       val C = new <parents> { <body>; def enumTag = n; $values.register(this) }
+
+   where `n` is the ordinal number of the case in the companion object,
+   starting from 0.  The statement `$values.register(this)` registers the value
+   as one of the `enumValues` of the enumeration (see below). `$values` is a
+   compiler-defined private value in the companion object.
+
+7. A simple case
+
+       case C
+
+   of an enum class `E` that does not take type parameters expands to
+
+       val C = $new(n, "C")
+
+   Here, `$new` is a private method that creates an instance of of `E` (see
+   below).
+
+8. A simple case consisting of a comma-separated list of enum names
+
+       case C_1, ..., C_n
+
+   expands to
+
+       case C_1; ...; case C_n
+
+   Any modifiers or annotations on the original case extend to all expanded
+   cases.
+
+## Translation of Enumerations
+
+Non-generic enum classes `E` that define one or more singleton cases
+are called _enumerations_. Companion objects of enumerations define
+the following additional members.
+
+   - A method `enumValue` of type `scala.collection.immutable.Map[Int, E]`.
+     `enumValue(n)` returns the singleton case value with ordinal number `n`.
+   - A method `enumValueNamed` of type `scala.collection.immutable.Map[String, E]`.
+     `enumValueNamed(s)` returns the singleton case value whose `toString`
+     representation is `s`.
+   - A method `enumValues` which returns an `Iterable[E]` of all singleton case
+     values in `E`, in the order of their definitions.
+
+Companion objects that contain at least one simple case define in addition:
+
+   - A private method `$new` which defines a new simple case value with given
+     ordinal number and name. This method can be thought as being defined as
+     follows.
+
+         def $new(tag: Int, name: String): ET = new E {
+           def enumTag = tag
+           def toString = name
+           $values.register(this)   // register enum value so that `valueOf` and `values` can return it.
+         }